Dimeric NF-:B transcription factors play critical roles in a wide variety of immune processes. Cytosolically sequestered in resting cells, NF-:B dimers are released to initiate gene transcription through the action of two basic signaling pathways known as the canonical and non-canonical NF-:B signaling pathways. These two pathways lead to release of NF-:B dimers on vastly different time scales and are regulated distinctly. While canonical NF-:B signaling is activated within minutes by a large number of receptors on a wide variety of cell types, non-canonical NF-:B signaling is activated over hours by a select group of receptors such as BAFF, LT2R, CD40 and RANK on a limited number of cell types such as B-cells, fibroblasts, and macrophages. However, both of these pathways play critical, non-redundant roles in the generation and survival of B-cells, osteoclasts, and secondary lymphoid tissues. It is now appreciated that hyperactivity of both canonical and non-canonical NF-:B signaling can lead to a variety autoimmune and proliferative diseases including Systemic Lupus Erythematosus, Multiple Myeloma, and Diffuse B-cell Lymphoma. While multiple negative feedback mechanisms have been described in the case of canonical NF-:B signaling which serve to terminate signaling and prevent pathological hyperactivation, no negative feedback mechanisms have been described for the non- canonical NF-:B signaling pathway. Our preliminary studies have demonstrated that IKK1, activated by NIK, can induce not only p100 phosphorylation and processing but also phosphorylation and destabilization of NIK. We have further mapped the IKK1 phosphorylation sites in NIK and shown that disruption of IKK1-dependent NIK phosphorylation can significantly increase NIK levels after receptor activation. Based on these preliminary results, we hypothesize that while previously reported TRAF-cIAP complex is responsible for NIK degradation in unstimulated cells, this novel IKK1-dependent NIK phosphorylation and destabilization mechanism plays an important negative feedback role in regulating non-canonical NF-:B activity after receptor activation. The goal of this R21 grant is to unravel the molecular components and mechanisms by which this feedback occurs. We propose to first identify the additional molecular components functioning within NIK-IKK1 feedback complex, then determine how these components assemble within the stimulus-induced NIK degradative complex, and finally determine the role of this feedback mechanism in regulation of both canonical and non-canonical NF-:B signaling. Together, we believe these studies will significantly enhance our understanding of non-canonical NF- :B regulation. Given the pathological potential of this pathway's hyperactivity in causing autoimmune diseases and cancers, further understanding of the molecular factors, biochemical relationships, and functional roles of negative feedback within the pathway will assist in future attempts to pharmacologically intervene in the pathway's activity. PUBLIC HEALTH RELEVANCE: It is now appreciated that hyperactivity of the non-canonical NF-:B pathway can lead to a variety autoimmune diseases such as Systemic Lupus Erythematosus and cancers including Multiple Myeloma and Diffuse B-cell Lymphoma. The goal of this R21 grant is to unravel the molecular components and mechanisms responsible for a novel feedback control pathway within non-canonical NF-kB signaling which we have identified based on our recent exciting preliminary results. We believe further understanding of the molecular factors, biochemical relationships, and functional roles of negative feedback within the non-canonical NF- :B pathway will assist in future attempts to pharmacologically intervene in treating autoimmune diseases and cancers.